Combat multiplier is increased when you deal damage to an enemy. The more damage you deal, the greater the increase. The maximum combat multiplier is limited to the amount of damage you are doing.

A good way to maintain a high combat multiplier is to use the spell Physical Shield and stock up on will potions. As long as this spell is active, your combat multiplier will not go down when you are hit, enabling you to achieve an extremely high combat multiplier when there are many enemies, or continually spawning enemies, such as the minions in the Focus Site quests right before the first fight with Jack of Blades. This is an easy way to cap your general experience (999,999) and master most or all of your abilities before the fight.

The combat multiplier has certain stages it reaches. The known stages are 5, 10, 20 and 40. When you reach a certain stage and take damage, your combat multiplier is reduced to the last stage. E.g. You have a multiplier of 9 and take damage, it is reduced to 5.

Changing areas or teleporting will also decrease the multiplier by approximately half.

Using Physical Shield will keep your combat multiplier from going down while it is active.

The experience multiplying can be exploited by getting it very high, then using an Ages potion. On newer characters, this can make you much stronger or skilled than you should be. (e.g. drinking a potion with a combat multiplier of 20 gives 20,000 experience)